1. Field of the Invention
The present invention is directed to a variable position gas trap apparatus and method used to separate gases entrained in drilling fluid in a tank. In particular, the present invention is directed to a variable position gas trap apparatus wherein a feedback control loop mechanically and automatically adjusts the height of the gas trap in response to changes in the level of the drilling fluid in the tank.
2. Prior Art
The use of drilling fluid or fluids while drilling subterranean wells is well-known. The drilling fluid or fluids may be aqueous-based, but are most often hydrocarbon or petroleum-based. The drilling fluids are referred to as base fluid, drilling mud or, simply, mud. Drilling fluid is used for a number of reasons. The drilling fluid is pumped downhole to the site where the drill bit is operating and is used to carry dirt, debris, rocks and chips broken off by action of the drill bit. The drilling fluid also assists in cooling the area where the drill bit operates. The drilling fluid may contain other additives, such as special lubricants, and is relatively expensive.
The drilling fluid is typically contained in a closed looped system. Upon return to the surface from downhole, the drilling fluid is often processed with a vibrating shaker or “shale shaker” which contains a screen so that the drilling fluid passes through the screen while rocks or other items above a certain size are separated out. The drilling fluid is stored in an open container or tank or a series of containers and then returned back down hole in a continuous system.
It has been discovered that the drilling fluid which returns from the downhole drilling location will return with downhole gas bubbles. The content of these gas bubbles provides extremely valuable information on the presence of hydrocarbons, such as natural gas. Monitoring of the gas content and composition as a function of depth is sometimes referred to as “mud logging”.
Assignee's U.S. Pat. No. 7,210,342 entitled “Method and Apparatus for Determining Gas Content of Subsurface Fluids for Oil and Gas Exploration” discloses one example of a system to analyze the gas content of bubbles entrained within the drilling fluid.
Over the years, there have been various devices that have been developed to liberate the gas bubbles which are entrained in the drilling fluid. Zamfes (U.S. Pat. No. 6,389,878) shows one example of a gas trap. A canister or container is partially submerged in the drilling fluid in the mud tank and permits drilling mud to enter from the base and exit from a side. The gas trap includes a motor which rotates a blade or stirrer to assist in releasing gas bubbles which are then taken to a gas collection port for analysis.
There are various types of gas traps, but most of them operate on similar basic principles. The gas traps are strapped or otherwise secured inside of the drilling mud tank. Changes in the operation of the drilling equipment or the drilling fluid pump can alter the level of fluid in the tank. If the drilling mud level in the tank or container changes the operation of the gas trap may be affected. If the level of the drilling mud is too low, not enough mud will enter the gas trap, so that primarily atmospheric air will enter the gas trap. If the level of drilling fluid is too high, it may affect the efficiency of separation of the gas bubbles from the drilling fluid or, in an extreme case, mud may enter the analysis equipment. While it is possible to manually move the gas trap in response to changes in the level, there is an ongoing effort to minimize required personnel at a drilling location.
Prior devices include Ratcliff (U.S. Pat. No. 4,358,298) which discloses a rack gear 66 that operates with a pinion gear 86 so that manual rotation of a crank 90 permits vertical adjustment of the gas trap. No automatic adjustment is provided.
Naess (U.S. Pat. No. 4,447,247) discloses a submerged mechanism to collect gas flowing into a body of water with an upper member 2 and ballast tanks 13 for adjusting the displacement of the upper member in an underwater blow-out.
Also in the past, a standard gas trap has been encapsulated in a buoyant sheath without any feedback control loop or mechanical assistance to respond to changes in the mud level. Despite the simplicity, the large footprint comprises its utility.
Notwithstanding the foregoing, it is desirable to provide a variable position gas trap apparatus wherein the position of the gas trap will automatically vary with the level of the mud in the tank.
It is also desirable to provide an apparatus that will operate with a wide variety of existing gas trap designs.
It is also desirable to provide a variable position gas trap apparatus having a feedback control loop for height adjustment.
It is also desirable to provide a variable position gas trap that is compact in design and reliable in operation.